Greenhouse Heating 



repairs. The gravity system of steam heatinj:^ fully meets the above-men- 

 tioned requirements. The apparatus is made automatic by draft regulators, 

 which are controlled by steam pressure. The regulators are adjusted to 

 different pressures, allowing a change of steam temperature to suit the 

 weather. Here the automatic feature ends. The fireman must do the rest, 

 keeping a clean fire, and replenishing with coal at the proper intervals. The 

 proper distribution of heat must be controlled by opening or closing the 

 valves until the required temperature is obtained. 



The installation of such a plant is comparatively simple, and does not 

 require more than ordinary mechanical knowledge. The constructor should 

 carefully consider the following salient points in making his layout for the 

 job : The boilers should be set with care, on solid foundations. A sub- 

 stantial chimney, with dimensions in proportion to the size of the grate, 

 should be provided. A good draught is of the utmost importance, as no 

 efficient heating apparatus can be maintained with a poor draught. The 

 steam mains should rise directly from the boiler, and pitch away from it, 

 and the size of the mains should be in proportion to the amount of radiating 

 surface carried in the coils. 



There should be no deviation from the rule which disallows any oppo- 

 sition of steam and water currents. All water of condensation should travel 

 in the same direction as the steam, and never against it. The lowest parts, 

 or ends, of the steam runs should not be less than twenty-four inches above 

 the water line of the boiler. If forty-eight inches can be obtained, so much 

 the better. The check valve on the return main at the boiler should be 

 of the best construction, and easy of operation, be kept properly cleaned 

 and in order. The radiation pipes are usually one and one-quarter inch 

 or one and one-half inch. The steamfitter should be careful to allow for 

 expansion of the pipes by heat ; otherwise, frequent leaks will occur at points 

 where this expansive strain exerts itself most. 



The Positive Return Method 



The second, or positive return system, differs from the first in that 

 the water of condensation is returned to the boiler by positive means, such 

 as a steam pump or injector. This system does not differ materially in 

 layout from the first, except that the factor of friction is not so important, 

 and the size of the steam mains is not so arbitrary. The steam mains may 

 be considerably smaller than in the ordinary gravity system. 



There are a number of plans employed in the installation of this sys- 

 tem. One plan is to carry a boiler pressure of one and one-half to ten 



183 



